A1201, A1202, A1203, and A1204
Continuous-Time Bipolar Switch Family
Functional Description
Bipolar Device Switching
The devices of the A120X family provide highly sensitive
switching for applications using magnetic fields of alternating
polarities, such as ring magnets. There are three switching modes
for bipolar devices, referred to as
latch, unipolar switch,
and
negative switch.
Mode is determined by the switchpoint charac-
teristics of the individual device. The characteristic hysteresis,
B
HYS
, of the device, is the difference in the relative magnetic
strength and polarity of the switchpoints of the device. (Note
that, in the following descriptions, a negative magnetic value
indicates a north polarity field, and a positive magnetic value
indicates a south polarity field. For a given value of magnetic
strength, B
X
, the values –B
X
and B
X
indicate two fields of equal
strength, but opposite polarity. B = 0 indicates the absence of a
magnetic field.)
Bipolar devices typically behave as latches. In this mode,
magnetic fields of opposite polarity and equivalent strengths
are needed to switch the output. When the magnetic fields are
removed (B
→
0) the device remains in the same state until a
magnetic field of the opposite polarity and of sufficient strength
causes it to switch. The hysteresis of latch mode behavior is
shown in panel A of figure 1.
In contrast to latching, when a device exhibits unipolar switch-
ing, it only responds to a south magnetic field. The field must
be of sufficient strength, > B
OP
, for the device to operate. When
the field is reduced beyond the B
RP
level, the device switches
back to the high state, as shown in panel B of figure 1. Devices
exhibiting negative switch behavior operate in a similar but
opposite manner. A north polarity field of sufficient strength,
> B
RP
, (more north than B
RP
) is required for operation, although
the result is that V
OUT
switches high, as shown in panel C. When
V
S
VCC
R
L
Sensor Output
A120x
VOUT
GND
(D)
(B)
(C)
V
CC
V+
Switch to High
Switch to Low
(A)
V+
Switch to High
Switch to Low
V
CC
V+
Switch to High
Switch to Low
V
CC
V
OUT
V
OUT
0
V
OUT(SAT)
B
RP
B–
0
B+
B
OP
0
V
OUT(SAT)
B
RP
B– 0
B+
B
OP
V
OUT
0
V
OUT(SAT)
B
RP
B–
0
B+
B
OP
B
HYS
B
HYS
B
HYS
Figure 1. Bipolar Device Output Switching Modes. These behaviors can be exhibited when using a circuit such as that shown in panel D. Panel A
displays the hysteresis when a device exhibits latch mode (note that the B
HYS
band incorporates B= 0), panel B shows unipolar switch behavior (the
B
HYS
band is more positive than B = 0), and panel C shows negative switch behavior (the B
HYS
band is more negative than B = 0). Bipolar devices,
such as the 120x family, can operate in any of the three modes.
A1201-DS, Rev. 2
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
8
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
